145 showed bimodal distributions when data was aggregated by fishing season and location (Hartigan’s dip-test;
146 DMale = 0.95, DFemale = 0.04, p-value < 0.001). The majority of male prawns and the smaller sized cohort of
147 female prawns caught in the small mesh science pots were of a size that would be discarded using the 10
148 mm riddle employed by Caridgan Bay fishers (Fig 2).
152 cohort analysis (1+ and 2+; summary statistics and ANOVA results in Table 2a). Table 2b compares
153 dimorphism highlighted by Forster (1951) and the present study. A higher proportion of the males (78.3 %)
154 caught were smaller than 10 mm CW compared to the females (39.7 %) in catches.
156 The maximum size observed in the sampled population showed females grew to a size considerably greater
157 than males, whilst the length distribution of catches show that the average male prawn within the 2+ cohort
158 does not reach a size at which it recruits into the Cardigan Bay prawn fishery.
159
160 Sex ratio
161 The sex-ratio of catches varied significantly from the expected 1:1 ratio, with both male and female directed
162 skews being observed throughout the sample period (Fig 3a)
163 164
6
P serratus biology & size-selective fishing
165 For all locations sex-ratios were female skewed in autumn and winter samples, with a higher proportion of
166 males caught in spring. Where an extended time-series was available from a single location, data exhibited
167 strong temporal trends in the sex-ratio and declining abundance of females as the fishing season progressed
168 in New Quay (3b); however, the data trend was less clear in the samples from Aberystwyth (Fig 3c).
169 Size at maturity (SOM)
170 Using an iterative search procedure, an inflection point was detected in the linear relationship between CL
171 and PLW in the female dataset. The data suggests that for pleura morphometrics, males display an isometric
172 growth pattern and females an allometric growth pattern. For females, the CL:PLW inflection point was
173 detected at 12.5 mm CL (Fig 4).
174
175
176 Size at maturity (L50)
177 Maturity is expressed as L50, which is the size (CL) at which 50% of the females were observed to be gravid
178 (carrying eggs). The maximum likelihood estimate of L50 estimated by the generalised linear model with a
179 binomial distribution was 15.9 mm CL (upper and lower confidence intervals = 16.4 mm and 15.4 mm CL
180 respectively; Fig 5).
181
182
183 Fecundity
184 Of the 616 gravid prawn that were captured by scientific pots, 273 (44%) were analysed for fecundity using
185 the equation described (1). Prawn ranged in size from 14.2 mm to 25 mm (CL) and produced fecundity
186 estimates of between 221 and 5,121 eggs per animal .
187
188 A Spearman’s correlation was run to assess the relationship between CL and fecundity. There was a strong
189 positive correlation, which was statistically significant (rS = 0.48, p <0.001) and is explained by the power
190 relationship below (figure6; equation 5). The fecundity data exhibits a high degree of variability with CL 7
P serratus biology & size-selective fishing
191 explaining just 22.2% of the variation in fecundity. Data points shown as triangles represent available
192 fecundity data from Forster (1951).
193 Fecundity = 92.546 푒0.1465 퐶퐿 (5)
194
195 DISCUSSION
196 Sexual dimorphism
197 Our results confirm that P. serratus in Cardigan Bay are sexually dimorphic, with females occupying a
198 broader length-distribution than males in the sampled populations. These results mirror the sexual
199 dimorphism that has been reported elsewhere for Palaemon serratus (Guerao & Ribera, 2000) and many
200 other Palaemon spp, with typically slower growth rates and smaller sizes in males. (e.g. Berglund, 1981; Ito
201 et al., 1991; Bilgin et al., 2009; Al Maslamani et al., 2013).
202 The evolutionary cause for dimorphism in this species is likely to have resulted from selection based on the
203 differing reproductive roles of the sexes (Shine, 1989). The current size-selective exploitation and resulting
204 pressure on mature females could potentially result in evolutionary responses that change growth and
205 reproductive patterns at a genetic level (e.g Conover & Munch, 2002; Walsh et al., 2006; Swain et al., 2007).
206 Given the short life span of P. serratus, fishery-induced responses such as decreasing size-at-maturity and
207 size-at age may occur over a timescale of years or decades (Reznick et al., 1997; Thompson, 1998; Koskinen
208 et al., 2002; Stockwell et al., 2003), a phenomenon that has been demonstrated in a number of other
209 exploited populations (e.g Grift et al., 2003; Olsen et al., 2004; Barot et al., 2004). Indeed, the selection
210 pressure towards large females and potential decrease in growth rates may have a negative effect on the
211 value of the species in the long term, which runs contrary to the larger prawns desired by the market. Hence
212 it is important to continue monitoring these life history characteristics, in order to determine any long term
213 changes, particularly in females. It would also be valuable to compare populations with varying degrees of
214 commercial exploitation.
215 Length frequency and commercial fishing
216 At present, the fishery is not subject to any statutory harvest-control-rules or technical-measures that aim to
217 encourage sustainable exploitation of prawn populations in Welsh waters in addition to the requirement for
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P serratus biology & size-selective fishing
218 commercial fishers to hold a shellfish license. The voluntary riddling of catch using either a 10 mm riddle or
219 > 10 mm pot mesh by some fishers ensures both a better market price and may return as many as 40% of
220 females to sea including sexually immature individuals. Since the grading of prawns is an entirely voluntary
221 practice, it is not possible to determine the relative proportion of common prawn landings in the UK that
222 have been graded at sea, on the quayside or not at all. Whilst the mortality rate amongst prawns discarded at
223 sea is still to be determined, personal observations indicate a very high level of mortality when prawns are
224 graded on the quayside. The absence of information on discard mortality rates calls into question the real
225 value of the riddling practice, particularly since the mortality rate is likely to be high. If the rate of mortality
226 amongst discarded prawn is at a significant level, the sex-specific consequences of riddling may not be as
227 severe as the data suggests. Nonetheless, there is a need to ensure that riddling is done at sea over fishing
228 grounds and habitat from which the prawn were removed. Some fishermen argue that a larger mesh size on
229 the fishing gear is a more appropriate conservation measure. We suggest that a gear comparison trial be
230 conducted to determine the gear design that maintains catchability whilst promoting the escape of undersized
231 prawn. Importantly the interaction between riddling and size-selectivity (i.e that a riddle will retain prawn
232 only of a size ≥ 10 mm) is an assumption in this study and not empirically validated. Future research needs to
233 collect data on retention rates of a known size distribution of animals being graded in order to evaluate the
234 real the real size and sex specific implications of the technical measure.
235 In 2008, when voluntary measures were adopted by some Cardigan Bay fishers, it was hoped that the
236 discarding of small prawn at sea would provide additional ecological and economic value by improving
237 market prices and releasing immature prawns to improve yield-per-recruit and spawner-per-recruit
238 respectively (CBFA pers. comm.). Our results show that by applying a size-selective harvesting regime, the
239 Cardigan Bay prawn fishery subjects the female population to a much higher level of removal relative to the
240 male population. Indeed, the immediate consequence of the quasi-minimum-landing-size would have been
241 the discarding of approximately 78% of male prawns caught in pots, compared to a female discard rate of
242 approximately 40% on average throughout the fishing season. The bi-modal distribution of size-frequency
243 data was present in all spatial and temporal combinations, representing a strong indication that two cohorts of
244 prawns are present during fishing the season. With the assumption that commercial activities select prawn at
245 a size 10 mm CW under the voluntary MLS, data shows females are recruiting into the fishery in their
9
P serratus biology & size-selective fishing
246 second year at a mean size of 12.32 mm CW (SE ± 1.22). However, fewer 2+ males are recruited into the
247 fishery, as the average male in their second year is 9.64 mm CW (SE ± 0.84).
248 Our study shows similar patterns in length-cohort distribution to previous studies. Forster (1951) reported
♀ ♂ 249 female population attaining a greater modal size than males within the 2+ cohort (TLMo ≈ 92.5 mm; TLMo ≈
250 77.5 mm). The above values are comparable to those reported in this study; however, the historical data
251 indicates a smaller average size of prawn within the 1+ group than we observed in this study (see table 2b),
252 although the difference is unlikely to be significant. The difference in 1+ size is likely to be as a consequence
253 of differing sampling methods employed by the two studies; Forster (1951) using fishery independent trawl
254 surveys in contrast to the present study, which used fishery-dependent ‘science pots’, which were fished
255 alongside commercial gear and therefore targeted the larger prawns.
256 Sex bias in the fishery
257 A consequence to size-selective fishing and higher rates of removal of female prawn may be evident in the
258 temporal trend of sex-ratios (Figure 3a), representing sex-overfishing on a regional scale. However, Fig 3b
259 and 3c shows that decreasing catchability of female prawn is location specific, with samples from
260 Aberystwyth showing a near 1:1 sex ratio late into the fishing season in comparison to fishing grounds to the
261 south, although the proportion of females in spring is still lower than during winter. The decreasing
262 abundance of females in catches marks the end of the prawn season as it is perceived by fishers as a
263 weakening fishery that yields less marketable catch. Seasonal variation in sex-ratios have been observed in a
264 range of palaemonid species (see Kim, 2005; Al-Maslamani et al., 2013) and has been attributed to
265 differential migration patterns, seasonal habitat preferences and possibly mortality between males and
266 females (Berglund, 1981). Female P. serratus are known to migrate between habitats to release larvae in
267 Wales (Haig et al. unpublished data) and hence it is unsurprising that we observes temporal and spatial
268 changes in sex-ratio as the fishing season progressed in Cardigan Bay as this may reflect localised
269 differences in timing of migration or habitat availability.
270 On a regional scale, fishing behaviour follows an inter-annual pattern whereby fishers in the south
271 experience the onset of the fishing season, with fishing opportunities gradually opening in a northward
272 direction along the Cardigan Bay coast (pers. obs. and CBFA pers comms.). Similarly, fishing opportunities
273 decline earlier in the south relative to the north, with fishermen from Aberystwyth and Aberdovey continuing
10
P serratus biology & size-selective fishing
274 to fish for months after fishing has ceased to be commercially viable in Fishguard and New Quay (pers. obs.
275 and CBFA pers. comm.). Fishermen therefore hold the view that females migrate in a northerly direction,
276 sustaining different rates of catch in different areas through the season. The scientific evidence presented
277 here neither validates nor disproves this view on the migratory behaviour of prawn in the region. Further
278 fisheries independent research (ideally using mark recapture methods) is required to determine if the
279 observed patterns in female catch indicate sex-overfishing, decreasing catchability as a result of seasonal
280 migration by females, or a cumulative response to both of these.
281 The potential for sex-overfishing identified by this study may have consequences on recruitment levels in the
282 future, although the life-history of palaemonids (highly fecund and typically multiple broods per season) may
283 safeguard it against depletion events. The data show female skewed catches in the early period of both
284 fishing seasons (Emmerson et al., 2014), which indicates the population has a degree of resilience in
285 sustaining size-selective fishing at present effort levels, the research presented here cannot draw a conclusion
286 with regards to sex-overfishing in the absence of both long-term datasets and evidence pertaining to adult
287 migration patterns.
288 Size at maturity
289 Crustacean fisheries are most commonly managed in the UK using a minimum landing size (MLS),
290 appropriated by maturity characteristics. In order to determine a valid MLS in decapod crustaceans , maturity
291 indicators such as morphological sexual maturity and functional maturity can be applied (Waddy and Aiken,
292 2005; Pardo et al., 2009). Size at maturity has been determined from allometric growth parameters (e.g.
293 Hartnoll, 1974; Little & Watson III, 2005; Claverie & Smith, 2009) and specifically the CL:PLW
294 relationship in Palaemonidae species (Cartaxana, 2003). In this study, the pleura has been shown to undergo
295 allometric growth in female P. serratus, which expand the brood chamber in preparation for egg carriage at a
296 size CW = 7.7 mm. At this point, females undergo an expansion in the PLW relative to males as they
297 continue to grow. It is highly likely that this dynamic allometry amongst females represents a physical
298 change of the abdomen in preparation for egg bearing and thus a sign of sexual maturity. Only 1.5% (n = 9)
299 of ovigerous females were observed at a size below our estimate of morphological size at maturity, implying
300 a high degree of confidence in the results of the iterative search procedure used. A total of 18.6 % of females
301 (n = 361) captured by scientific pots throughout this study had a CW < 7.7 mm and were assumed to be
11
P serratus biology & size-selective fishing
302 sexually immature. With a CW < 10 mm, immature female prawn that have yet to develop their brood
303 chamber and are released by Cardigan Bay fishers onto the fishing ground from where they were captured.
304 The size at morphological sexual maturity supports the results from this study’s estimate of functional
305 maturity (L50), with the results implying that female prawn undergo a puberty moult at an estimated size CW
306 = 7.7 mm, whilst 50% of females are able to contribute to the reproductive capacity of the population by the
307 size CW = 9.9 mm (15.9 mm CL). In this way, the voluntary measure of releasing prawn below CW = 10
308 mm by CBFA fishers has been shown to be a potentially valuable conservation measure. The CL50 reported
309 here is greater than that reported in similar studies elsewhere for the species (Ireland; CL50 = 12.5mm; Kelly,
310 2009), though similar to previous estimates for the Welsh population (CL50 = 16.5; Huxley, 2011).
311 Fecundity
312 P. serratus were found to carry between 221 and 5,121 eggs at any one time (mean average = 1,916). This is
313 similar to estimates published by Forster (1951), who found large prawn (TL = 105 mm) carry up to 4,282
314 eggs and within a similar range of other Palaemonidae species (Corey & Reid, 1991). The fecundity (number
315 of eggs carried) of female prawns was positively correlated with body size (CL); however, there was a high
316 degree of variability between individuals and CL only explained ~23% of the variation. Studies of similar
317 species (P. elegans, P. adspersus and P. xiphias) report R2 values > 0.95 (Guerao et al., 1994; Cartaxa, 2003;
318 Bilgin & Samsun, 2006). Different methodologies for estimating fecundity may be the reason behind the
319 variable R2 values reported here and in the published literature. In particular, previous fecundity estimates
320 were derived from the number of eggs at stage 1 (e.g Guerao et al., 1994) in order to account for egg loss
321 during incubation, which can be the result of mechanical stress or parasites (Glamuzina et al., 2014) and has
322 been reported to be as high as 38% in this species (Reeve, 1969 in Zimmermann et al., 2015). Egg counts by
323 developmental stage were unavailable in this study, which is the likely explanation for the high variability in
324 the fecundity estimate. Nonetheless, the results are within the range reported for the species, as shown in
325 figure 6, and provide an important baseline from which to further understand the reproductive capacity of the
326 Welsh P. serratus population by providing an estimate between the numbers of eggs laid on pleopods during
327 spawning and the total that eventually hatched.
328 Fecundity can be influenced by temporal-spatial variations of environmental factors such as depth (e.g P.
329 naval; Thessalou-Legakiand, 1992), mean bottom temperature (e.g P. borealis; Parsons & Tucker, 1986) and
12
P serratus biology & size-selective fishing
330 habitat (e.g E. modestus and P. gravieri; Oh & Park, 2000). We recommend future studies pay particular
331 attention to the problem of egg loss during brooding on pleopods by staging eggs using the criteria outlined
332 in Guerao and Ribera (1995). Preservation of samples would permit a more accurate estimate of real
333 fecundity, which should incorporate egg stage, size-dependent egg losses and egg-quality into the fecundity
334 estimate. The limited scope and resources dedicated to the present study has constrained the data available
335 for our fecundity estimate; however, it provides a useful baseline from which to continue monitoring.
336 Management
337 The aim of this research was to provide a series of region-specific indicators that can be used by fisheries
338 managers in the Cardigan Bay prawn fishery to guide biologically appropriated management measures. The
339 voluntary measures employed by some fishing industry members in Cardigan Bay are effective at protecting
340 50 % of the female brood stock in their catch providing the discard mortality rate is low. The process of
341 grading prawn on the deck of a commercial fishing boat can be resource intensive and fishers have
342 engineered bespoke riddle systems or replaced gear for larger mesh traps that may increase the efficiency of
343 the gear in selecting larger prawn. While the results presented here demonstrate the potential ecological
344 benefits of using either a larger mesh size, or a riddle in the prawn fishery, many commercial operations do
345 neither (CBFA pers. comm.). Given the potential economic and ecological value of increasing the size prawn
346 reaching the market and the reluctance of some industry members to alter their fishing strategy, a
347 comprehensive analysis of technical options should be explored. This might include the effectiveness of
348 escape panels and minimum mesh-sizes (Fothergill, 2006), which would allow animals to escape before
349 being exposed to increased stress and mortality rates associated with handling.
350 The limited evidence presented here suggests that there has not yet been an observable effect of overfishing
351 on size-at-maturity of females in Cardigan Bay. However, declining CPUE in other exploited populations on
352 the coast of Ireland suggest that overfishing can occur (Fahy & Gleeson, 1996) and hence long term
353 monitoring of any changes should be included as part of ongoing fisheries management. At present, there are
354 no statutory requirements to collect size at maturity data on P. serratus despite it being recognised as an
355 important parameter for fisheries management. Ideally, these investigations would be replicated at an
356 appropriate temporal scale; and given the short-life span of P. serratus, we recommend biennial replication.
357 Given the potential for sex-overfishing within a size-selective harvesting regime in the Welsh prawn fishery,
13
P serratus biology & size-selective fishing
358 the extent of which is yet to be fully understood, affording a scientifically-validated level of protection to
359 juvenile females via a MLS would be a valuable safeguard against recruitment failures in the future.
360 ACKNOWLEDGEMENTS
361 This project would not have been possible without the help of the Welsh Fisherman’s Association,
362 particularly Jim Evans, who facilitated our introduction to many fishers who collaborated with the study.
363 Similarly, we wish to thank Huw Evans of the Cardigan Bay Fisherman’s Association who supervised the
364 project and provided valuable insight from an industry perspective. We thank the following fishers for their
365 invaluable input to this research: Llew and Ryan Hughes, Chris Hicks, Dean Parry, Mark Hughes, David
366 ‘Sturdy’ Jones, David Kendall and Paul Walsh. Finally, the authors extend their gratitude to Dr. Vlad
367 Laptikhovsky and the two anonymous reviewers who highlighted revisions that greatly improved this paper.
368 FINANCIAL SUPPORT
369 This project was funded by the Cardigan Bay Fishermen’s Association via the European Fishery Fund (EFF)
370 and the Welsh Government.
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510
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511 FIGURE LEGENDS
512 Fig 1 The homeports for the six active Palaemon serratus fishers in Cardigan Bay, Wales; who 513 contributed monthly samples (when possible) during the prawn fishing seasons from 2013, 2014 514 and 2015. Ports are numbered north to south and are as follows: 1, Aberdovey (2 fishers); 2, 515 Aberystwyth; 3, New Quay; 4, Cardigan; 5, Fishguard
516 517 Fig 2 A length frequency histogram with a probability density function for male (above) and 518 female (below) Palaemon serratus caught in science pots during the 2013-2015 Palaemon serratus 519 research period in Cardigan Bay. The solid vertical red line represents the voluntary sorting size 520 (10 mm CW) used by many fishers in Cardigan Bay
521
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522 Fig 3a The sex-ratio of prawn (Palemon serratus) caught in science pots from during the 2014 / 523 2015 in Cardigan Bay and 3b the sex ratio of catches in localised datasets from New Quay and 3c 524 Aberystwyth
525 526 Fig 4 Inflection point indicating allometric growth based on morphometric variance between 527 iterative tests on linear models of PLW and CL for the prawn Palaemon serratus. The dotted 528 vertical line is the value with the lowest mean standard error (12.5 mm CL). Solid black line shows 529 the linear male relationship. Hashed line shows the allometric female relationship after inflection 530 event
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P serratus biology & size-selective fishing
532 Fig 5 Functional maturity model fit for female prawn (Palaemon serratus) from Cardigan Bay 533 (Wales) with 95% CIs as indicated by the presence or absence of eggs. The horizontal line 534 represents L50 (15.9 mm CL) for the females sampled within period of peak spawning (April; n = 535 544)
536 537 Fig 6 Fecundity of gravid prawn (Palaemon serratus) from Cardigan Bay (Wales) with size (CL) 538 (n = 273). The solid line shows the power relationship between the correlating variables CL and 539 Fecundity (p < 0.001; R2 = 0.222). The red triangle points and associated hashed trendline show 540 the fecundity data available from Forster (1951).
541
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